groups of threes to make p+, e-, or n0History of the Atomic ModelRutherford ExperimentElectron Cloud Model-1926

“Probability Cloud”

Electrons orbit the nucleus in energy levels or orbits.

Why is it considered a “probability cloud”?

EVOLUTION OF ATOMPERIODIC TABLEThe number of neutrons = mass number –atomic number

In a neutral atom the number of e- and number of p+ are equal

Every atom of the same element has the same number of protons

MASS NUMBER =ATOMIC MASS

ISOTOPES

Atoms of the same element with different numbers of neutrons

Atomic Mass Units

Do we now know how much atoms really weigh, in pounds or grams?

Yes, but we don't normally use those units for measuring the mass of an atom. It's much more convenient to use something called the atomic mass unit, or amu.

Using AMU is simplier!

One gram is about 600,000,000,000,000,000,000,000 amu (that's 600 sextillion, or a 6 followed by 23 zeros). A pound is just shy of 300 septillion amu--that is, 300,000,000,000,000,000,000,000,000. (26 zeros there.)

Periodic Table

1869 Dmitri Mendeleev arranged the elements in order of their increasing atomicmasses

He discovered a pattern and was able to give rise to the modern

periodic tableMendeleev’s Periodic TablePERIODIC LAW

The properties of elements are a periodic function of their atomic number

The properties change in a repeated pattern as we move across or down the periodic table

Elements are arranged by changes in physical and chemical properties

MODERN PERIODIC TABLE

1913 Henry G.J. Moseley

Arranged the periodic table based on increasing atomic numbers

PERIODS or Series –horizontal rows 1-7

GROUPS or Families-vertical columns

1-18 with similar propertiesProperties of Groups

Elements of same group have same # valence electrons

Valence electrons- are the e- in the outermost energy level

Important role in chemical reactivity

Electrons closer to the nucleus have lower energy than those farther away

Properties of SeriesIonization Energy

Amount of energy needed to remove the outer electrons

As you move right to left the radius of the nucleus increases meaning the size(distance) between the nucleus and electrons is greater

As you move left to right the radius decreases meaning the electrons are valence electrons closer to the nucleus, held tighter and require more energy to remove

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